1. Field of the Invention
This invention pertains to a drawing die, and more particularly to a nib for a drawing die and method wherein subsequent nibs recut from the original nib maintain geometric similarity.
2. Description of the Prior Art
Typically, a drawing die comprises a casing and a nib secured in the casing, and the incoming material to be drawn is drawn through and reduced in transverse dimension by the nib. The nib is generally made of a hard substance, such as carbide, natural diamond, manufactured diamond and the like. The nib has several cavities through which the incoming material is passed to be drawn. Those cavities are in sequence the entrance cavity, reduction cavity, bearing cavity, and exit cavity. The entrance angle of the entrance cavity is greater than the reduction angle of the reduction cavity, and the drawing of the incoming material is performed in the reduction cavity. The bearing cavity is generally of cylindrical shape, and the exit cavity angles outwardly from the bottom of the bearing cavity.
In the drawing process, the size of the nib, i.e., the size of the cavities, is determined by the size of the incoming material to be drawn so that the impact plane generally lies transversely approximately midway in the reduction cavity. Typically, it is desirable that the material is drawn through about 50% to 60% of the reduction cavity. The bearing cavity size determines the ultimate transverse dimension of the drawn product.
After prolonged use, the reduction cavity and bearing cavity tend to wear and become unusable in producing a satisfactory drawn product. The worn nib is then recut to a larger size for use with larger incoming material to be drawn. It is with the recut nib that problems arise in the prior art. For example, most nibs have an entrance cavity with a curved sloping surface that tapers inwardly and downwardly to the reduction cavity. Upon being recut, the length of the reduction cavity is increased, thereby decreasing the length of the entrance cavity. This reduction of the length of the entrance cavity presents the undesirable condition of effectively lowering the position of the impact plane in the reduction cavity to a point where only about 30% of the reduction cavity is used for drawing the material.
Another problem is that upon being recut, a sharp edge is formed at the transition of the entrance cavity and reduction cavity which, if not refinished, blended, or polished, can scratch or mar the surface of the incoming material to be drawn, and consequently produce a scratched or marred product. The scratch or mar is not removed during the drawing process, but is lengthened in the drawn product. To avoid this problem, the sharp edge can be refinished, but this increases production cost.
Still another problem is that two nibs can be purchased for drawing the same size incoming material, but which can differ in shape or geometry in the entrance cavity, exit cavity, and the upper portion of the reduction cavity. The resulting recut nibs generally bear no geometric similarity in appearance to the original nib or other recut nibs, a result not desired in the industry.
A further problem associated with the prior art nib is the increased difficulty and cost in machining the curved inner surface of the entrance cavity.
Attempts have been made to rectify the above problems, one of which is to provide the entrance cavity with a straight-line or conical surface instead of a curved surface. Although this somewhat alleviates a few of the above problems, it does not eliminate them.
As prior art nibs continue to be recut, the geometry of the cavities and bearings become undesirably distorted. Further, there is no disclosure, to the best of applicant's knowledge, in the prior art of any formulas, equations, or the like, which can be used in a process or method of manufacturing a nib which, upon being recut, yields a recut nib substantially maintaining geometric similarity with the original nib.
In view of the above, there clearly exists a need for a nib, and a method of its production, which upon being recut substantially maintains a desired geometric relationship between the cavities so as to produce acceptable drawn products.